One of the biggest problems in managing fuel cell systems (especially high temperature fuel cell systems, such as solid oxide fuel cell (SOFC) systems) is avoiding oxidation of the anode electrodes (i.e., the fuel electrodes). Oxidation of the anode electrode occurs when the oxygen partial pressure in the anode chamber increases to a point where the metallic anode is no longer stable. One commonly used metal in SOFC anodes is nickel which can form nickel oxide unless a reducing environment is maintained during SOFC operation.
Oxidation of the anode electrode generally causes a performance loss or degradation of the fuel cell. This performance loss is caused by the volume change of the nickel transitioning to nickel oxide. Upon re-reduction of the anode electrode (reversing the volume increase), the microstructure of the anode shows fractures which reduce the percolation of the nickel network in the anode and thereby limit the electrical conductivity of the anode. Depending on the nature of the oxidation and re-reduction, as well as the composition and microstructure of the anode, the damage due to oxidation can be anywhere between small and catastrophic.
Oxidation of the anode electrode commonly occurs during a fuel outage or shortage while no current is flowing or while current is flowing. Normally when fuel becomes unavailable, the fuel cells are taken off current and the anode chambers are purged with a reducing gas to prevent oxidation. This purge flow has to be maintained while the fuel cells remain hot enough to oxidize. Significant volumes of purge gas may be required for the purge. In many fuel cell installations, the volume of the purge gas storage approaches or exceeds the volume of the fuel cell power generator itself. Advances in anode structure and composition have be made towards reducing the impact of oxidation, but nevertheless losses still occur and should be avoided.